1. Field of the Invention
The present invention relates to a method for detecting precipitation using a radar locating device for motor vehicles that is designed to locate objects in the surroundings of the vehicle, in which method a locating signal, which is an index for the received power density as a function of the distance, is integrated over a first distance range that is below a limit distance for the detection of precipitation.
2. Description of Related Art
Radar locating devices are frequently used in driver assistance systems for motor vehicles, for example, for an automatic distance regulation or for the early detection of a risk of collision.
Precipitation in the form of rain or also spray spattering up from the road may reflect a portion of the emitted radar radiation and thus cause a reduction of the range of the radar radiation and thus a reduction of the locating depth of the radar sensor. For reasons of traffic safety, it is important that it is possible to detect such a restriction of the function of the radar locating device preferably without delay.
Published German patent application document DE 10 2006 054 320 A1 describes a method of the type mentioned at the outset for a multibeam radar, in particular for an FMCW radar locating device. In this method, the locating signals of a plurality of radar beams are respectively separately integrated, and the resulting integrals are compared to one another.
The functional principle of an FMCW radar locating device (Frequency Modulated Continuous Wave) consists in the radar signal being continuously emitted; however, the frequency of this signal is periodically modulated with rising and falling ramps (in this instance, the term “ramp” is not to mean that the frequency modulation within the “ramp” necessarily has to be linear.) A mixer mixes a portion of the transmission signal with the signal received by the antenna and thus generates a mixed product whose frequency corresponds with the difference between the frequency of the current transmission signal and the frequency of the received signal.
If a radar echo from a located object is received, the frequency of the mixed product is thus a function of the signal propagation time and thus the distance of the object, but, due to the Doppler effect, it is also a function the relative speed of the reflecting object. Thus, in the spectrum formed from the mixed product, each located object emerges on each modulation ramp as a peak in the frequency that is a function of the distance and the relative speed. The distance and the relative speed of the object may then be determined by comparing the frequency positions of peaks that derive from the same object, on modulation ramps having different slopes.
Rain drops or spattering spray also constitute “objects” in this sense, which leave a weak but still detectable peak in the spectrum when the radar sensor is not at too great a distance, at distances up to approximately 10-50 m, for example. In the event of heavy precipitation, these peaks in the frequency range, which corresponds to the above-mentioned distance range, add up to a background signal, the so-called rain clutter. The power of this rain clutter is thus an index for the presence and the strength of precipitation.